A major use for the epoxy resins is as surface coating materials. They combine toughness, flexibility, adhesion, and chemical resistance to a very desirable degree. However, they have inherent limitations that have restricted their use.
Coatings that contain no solvents have been prepared from very low molecular weight epoxy resins. The resin itself, in such coatings, serves as the wetting agent and as the vehicle for any pigments or fillers desired. Because there is no solvent, coatings of this type tend to be free from pinholes, but they have disadvantages such as brittleness, poor thermal stability, relatively high cost, and short pot life.
Coating compositions based on higher molecular weight epoxy resins have been prepared as solutions, formulated with solvent vehicle, curing agents and modifiers, and often, with pigments and opacifiers. The epoxy resin is often in the form of an ester, obtained by reacting the epoxy resin with a fatty acid, drying oil, or the like. While suitable for many purposes, expoxy ester-based coatings are vulnerable to caustic attack. The ester linkages are not considered to be as stable as would be desirable, for many applications.
In recent years there has been a trend toward water-based coating compositions containing epoxy resins, because of ease of handling and cleanup. Many attempts have been made to develop such coatings, and some of these have met with some success for particular applications. One promising area for the possible application of such coatings is for soft drink and beer cans. This application has always presented a challenge because of taste sensitivity. Can coatings in the past have tended to alter the product taste of canned beverages, in a variety of ways, sometimes by leaching of coating components into the beverage, sometimes by adsorption of flavor notes by the coating, sometimes by chemical reaction, and often by some combination of these. There is a commercially important, technically challenging, large potential application in can coatings for a water-based coating composition that is chemically stable, absolutely inert in taste response, easy to apply, and economically competitive, and that produces coatings that have all of the other demanding characteristics that are associated with that application, as reflected in the many prior art attempts to develop satisfactory products.